Cost Management Systems

1
E-R Diagram for a small manufacturer
2
Basic E-R Notation
3
E-R Model Constructs

• Entity - person, place, object, event, concept
• Entity Type versus Entity Instance
• Entity Type versus System Input, Output, or User
• Strong Vs. weak entity types
• Attributes
• Simple Vs. Composite Attributes
• Single-Valued Vs. Multi-valued Attribute
• Stored Vs. Derived Attributed
• Relationships

4
Example of inappropriate entities
System user (Treasurer) and output (Expense
Report) shown as entities
5
E-R Model Constructs

• Strong versus Weak Entity Type
• Identifying relationship.
• Weak entity identifier is its partial identifier
  combined with that of its owner.

6
Example of a Weak Entity
7
A Composite Attribute
8
Entity with a Multi-valued Attribute (Skill) and
Derived Attribute (Years_Employed)
9
Identifiers or Keys

• Identifier or Key - An attribute (or combination
  of attributes) that uniquely identifies
  individual instances of an entity type.
• Simple Key versus Composite Key
• Criteria for Selecting Identifier or Keys
• Will not change in value.
• Will not be null.
• No intelligent identifiers (containing e.g.
  locations or people that might change)
• Substitute new, simple keys for long, composite
  keys.

10
Relationships

• Degree of a Relationship - number of entity types
  that participate in it.
• Unary (or Recursive) Relationship
• Binary Relationship
• Ternary Relationship
• CAN have Attributes of Relationships

11
Degree of Relationship
Binary
Course
Student
Vendor
Unary
Warehouse
Supplies
Ternary
Employee
Manages
Part
12
Attribute on a Relationship
13
An associative entity (CERTIFICATE)
14
Supplies Relationship and Associate Entity Supply
Schedule
15
Relationships

• Cardinality Constraints - the number of instances
  of one entity that can or must be associated with
  each instance of another entity.
• Minimum Cardinality
• If zero, then optional.
• Maximum Cardinality
• Mandatory One - when min max both 1.

16
Basic Relationship
17
With Cardinality Constraint
18
Mandatory Cardinalities
19
One Optional, One Mandatory Cardinality
20
Optional
Unary
Employee
Manages
21
Cardinality constraints in a ternary relationship
22
Simple Example of Time Stamping
23
E-R Diagram Recognizing Product Reassignment
24
Professors and Courses
25
Summary

• ER model for conceptual data modeling is a
  logical representation of data for an
  organization
• Basic constructs are entity types, relationship
  related attributes.
• An entity can be a person place location, object
  event, or even a concept.
• A relation type is meaningful association between
  entities .
• Degree of relationship is the number of entity
  types involved in a relation.
• A cardinality constraint is a constraint that
  specifies the no. of instances of B associated
  with each instances of A.